This study examined the hierarchical structure among second/foreign language (L2) writing subskills using a Hierarchical Diagnostic Classification Model (HDCM). A pool of 500 essays composed by English as a Foreign Language (EFL) students was assessed by four experienced EFL teachers using the Empirically-derived Descriptor-based Diagnostic (EDD) checklist. Based on a literature review and the expertise of three content experts, several models were developed to reflect various hierarchical interactions among L2 writing subskills, including linear, divergent, convergent, independent, unstructured, mixed, and higher-order. The comparison of the models showed the presence of an unstructured interaction among L2 writing subskills, indicating that content is the foundational subskill for the mastery of vocabulary, grammar, organization, and mechanics. Higher mastery classes were also associated with higher educational levels, greater frequency of English use, and longer exposure to L2. Understanding the hierarchical relationships among L2 writing subskills can improve targeted instructional strategies and assessment practices. • A constrained version of existing DCMs is represented by hierarchical DCMs. • Models were developed to show hierarchical interactions among L2 writing subskills. • An unstructured interaction among L2 writing subskills was identified. • Higher mastery classes were associated with higher educational levels. • The classes were associated with greater English use and longer L2 exposure.